ground reference transformer sizing

8/13/2019 Ground Reference Transformer Sizing

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Ground Reference Transformer Sizing

Example 1:

1. Objective: The three-phase fault duty at a Wind Farm facility is 5kA (299 MVA) on the

ungrounded 34.5 kV system. It is required to ground this system using a zigzag groundingtransformer and limit the ground-fault current to 400 A. the objective of this study is specifying thegrounding transformer.

2. Input Data

a. System Voltage: 34.5kV, 3phase, 3Wire

b. System Grounding: Ungrounded

c. Type of required grounding Source: c1: zigzag transformer

c2: wye-delta

d. Neutral ground current that groundingtransformer shall carry under fault: 400A

3. Calculation:

a. Transformer Impedance

Use 100 MVA Base (100000 kVA)

Rated Voltage = 34.5 kV

Base Ohms = (1000) (kV)2/KVA=1000 (34.5)

2/100000= 11.9 Ω

Base Amps = KVA/(KV × √3)= 100000/(34.5 x √3)= 1673 A

Z 1 = Z 2 = 100/299 = 0.334 per unit (p.u.)

Positive

sequence0.334 p.u.

Negative

sequence0.334 p.u..

Zerosequence Z0

IG = 400/1673= 0.24 p.u. A

I0 = 0.24/3= 0.08 p.u. A

Z total = 1/0.08= 12.5 p.u. Ω


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Z total – 2 Z 1 = 12.5 – 0.334 – 0.334= 11.832 p.u. Ω

Z 0 = 11.9 × 11.832= 140.8 Ω/phase for grounding transformer

b. Transformer short time kVA rating

i. For a zigzag grounding transformer, since both windings are active in the primary circuit,

the actual (short-time) rating in kilovoltamperes is:

kVA= (Line-to-Line kV)(Neutral Amps)/3= (34.5 x 400)/3=4600 kVA

ii. For a wye-delta grounding transformer, the actual (short-time) rating in kilovoltamperes

is:

kVA= (Line-to-Line kV)(Neutral Amps)/√3= (34.5 x 400)/√3=7967 kVA (8 MVA)

Because a grounding transformer is a short-time device, its size and cost are less than for a

continuous duty transformer of equal kVA rating. The reduced size can be established in

terms of an “equivalent two-winding 55 C kVA” kVAx, by applying a reduction factor K to the

short-time rated kVA of the grounding transformer, and this reduced kVA can be used for a

price estimate.Since a grounding transformer is a short-time device and normally only required to carry

short-circuit ground current until the circuit breakers clear the fault, it is common to rate it

on a reduced time such as 10 s or 1 minute. Under these circumstances the physical size (and

resulting cost) is considerably reduced. The reduced size can be established in terms of an

“equivalent two-winding 55 C kVA” kVA reduced, by applying a reduction factor K to the short-

time rated kVA of the grounding transformer.

Values for K are listed in Table 1 below for various types and classes of grounding

transformers;

Table 1- K FACTORS FOR DETERMINING EQUIVALENT TWO-WINDING 56 C KVA OF

GROUNDING TRANSFORMERS


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Since in wind farms the 34.5 kV system could be subjected to more than one faults in quick

succession 1 minute rating is preferable.

i. The zigzag grounding transformer is required to continuously carrying the 34.5

kV line to-ground charging current and the unbalanced current (if any).

As per IEEE 32, the continuous current for a zigzag grounding transformer with a 1minute

time-rating is equal to the 7% of the ground fault current i.e.

Icontinuous= (7/100) x 400= 28 A

Continuous kVA rating= (34.5 x 28)/3=322 kVA

4. Proposed Zigzag Grounding transformer

Technical Requirement for the Proposed Grounding Transformer

Cooling ONAN

Installation Outdoor

Winding Configuration Zigzag

Rated Voltage 34.5 kV

Rated BIL 200 kV

Continuous Neutral Current 28 A

Rated Neutral Fault Current 400 A for 1 minute

Zero sequence impedance 140.8 Ω

Temperature Copper: 55⁰C, oil: 50⁰C at 40⁰C ambient temperature

Standard CAN/CSA C88-M90 (R2009)

Accessories

Normal Special


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1 Conservator with liquid drain valve and

filling valve

1 Buchholtz relay

2 Breather 2 Winding temperature indicator

3 Liquid level indicator 3 Conservator magnetic liquid level indicator

4 Liquid draining and sampling valve 4 Anti-condensate heater

5 Top oil thermometer 5 Pressure relief device

6 Lifting, moving detanking & jacketing

facilities6 4 CTs, 600/5A, C50, B0.5

7 Name plate 7 Control box

8 Grounding pad

5. Evaluation of Proposed Zigzag Grounding transformer

To examine the proposed grounding transformer is suitable it has to be shown that:

Z0 ≤ XC0

Where,

Z0 = Zero sequence impedance, and

XC0= line-to-ground capacitive reactance of the system

Or in another way, the current in the zero sequence impedance IG during a line-to-earth fault

must be equal to or greater than three times the line-to-earth system charging current, 3IC0

IG ≥ 3IC0

3IC0 could be calculated from below formula:

3IC0 = (√3) *(2 xπ x f x C 0 x V L-L ) /103 ] Amperes

Where,

VL-L = system line-to-line voltage in kilovolts,

C0 = zero sequence capacitance of the system,

f = system frequency

The zero sequence capacitance of any type of cable can be calculated using the following

formula:

C 0 = (0.02415 x SIC)/log (D/d) µF/ km

Where,

SIC = dielectric constant,

D = the diameter of cables over the insulation shield,

d = the diameter of the conductor,

The zero sequence capacitance of transformer is negligible. However, for overhead lines, zero

sequence capacitance can be high if considerable lengths are involved.


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As a general rule, the following approximate capacitance values could be used:

Transformer C0 = 0.01 µF/transformer

Overhead line C0 = 0.00625 µF/ km

Note: the given admittance of collector system, i.e. YC =170.5 micro-mho does not seem reliable;

however it meets the above mentioned requirement.

It is recommended to verify the line-to-ground capacitive reactance of the collector system

based on the formulas given above.

Mehrdad Boloorchi, P.Eng.

Associate, Senior Systems Engineer

ground reference transformer sizing

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